in the related art, a monopole antenna having a small size and a simple structure has been widely used as an antenna device of a mobile phone (refer to NPL 1). Hereinafter, an antenna device of a mobile phone that is mounted with the monopole antenna in the related art will be described referring to FIGS. 13(a) and 13(b).
FIGS. 13(a) and 13(b) show an antenna structure of a monopole antenna mounted to a mobile phone and electric current distribution in operation. In FIG. 13(a), a ¼λ monopole antenna 102 is mounted to a printed circuit board 101 of a mobile phone main body through a power supply point 103. In FIGS. 13(a) and 13(b), since the monopole antenna 102 resonates at a desired frequency by an antenna element and an antenna ground and forms a single antenna, a ground current ie over the printed circuit board 101 and the ground according to an antenna current ia flows in the printed circuit board 101, as shown in the figures.
In such a configuration, in terms of the size reduction for the SAR or the electromagnetic wave interference to the hearing aid, the antenna structure shown in FIGS. 13(a) and 13(b) has the following problem. In the ¼ monopole antenna, since the printed circuit board 101 of the mobile phone main body forms the ground, the ground current ie flows in the printed circuit board 101. The ground current intensively flows in the vicinity of the power supply point 103, but the portion where the electric current intensively flows is a portion where a head part is closest to a portion where a speaker 104 is installed. Thus, in the mobile phone mounted with the monopole antenna, it is difficult to reduce the SAR or the electromagnetic wave interference in the hearing aid.